1. Field of the Invention
The present invention relates to a speed detecting apparatus for vehicles and, more particularly, to a speed detecting apparatus which is capable of detecting speed and/or acceleration with high accuracy.
2. Description of the Prior Art
Speed V and acceleration .alpha. are very important factors for a vehicle in objectively grasping the situation in which the vehicle is traveling. The speed detecting apparatus of a vehicle is therefore required to detect the speed and the acceleration with accuracy.
Especially, in present-day highly-electronized vehicles, the data on the detected speed V and acceleration .alpha. are widely used for various purposes, for example, for controlling automatic door locks, a power steering, a digital speed controller, the ignition time of the engine, a navigation controller and so forth. Accurate detection of these data is therefore very important for improving the drivability and comfortableness in the vehicle compartment.
In particular, in a vehicle equipped with an electronic skid control system (a system for preventing the vehicle from skidding due to the lock of the wheels at the time of a sudden application of the brakes), the rotating speed and the acceleration of each of the four wheels in total, namely, front and rear, right and left wheels, are detected separately from those of the other wheels by using a speed detecting apparatus. The detected speed and acceleration and a minute change therein are compared with the ideal travelling conditions stored in a computer in order to control the braking force of each wheel. Accordingly, very high-accuracy real time measurement of the wheel rotating speed and the acceleration is required.
A conventional speed detecting apparatus, however, cannot always detect speed and acceleration in a wide range from a low speed to high speed (e.g., 2.5 km/h to 250 km/h) with such high accuracy as is required by the electronic skid control system (hereinunder referred to as "ESC system").
The reason thereof will be described in detail hereinunder.